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NICMOS Aperture Definitions
Each HST Science Instrument requires its own local coordinate system and apertures to support both target acquisition and small angle motions (SAMs). Apertures are calibrated locations in the HST focal plane relative to the FGS frame. All acquisitions and SAMs are relative to apertures. Any location within the field of view of a NICMOS camera can be specified by the POSTARG special requirement (described in the HST Phase II Proposal Instructions).
Aperture Definitions
The basic philosophy of the NICMOS aperture definitions follows that used by WF/PC-1 and WFPC2. Each NICMOS camera has two primary apertures. One is positioned at the geometric center of the detector and the other at an
optimal
position close to the center. The first of these apertures is anchored to that fixed location, while the second may be moved in the future. In this way the optimal aperture may be shifted to avoid array defects, even if these are time dependent. Observers with large targets which fill the field of view of a particular camera are generally advised to use the first type of aperture, while for observers with smaller targets the second type is recommended
Additional apertures are defined in Camera 2 for use in the Mode 2 coronographic acquisition.
Standard Apertures
The names of the defined apertures are listed in
Table 4.4
along with a description of their function and their initial location.
Table 4.4: NICMOS Aperture Definition
Aperture Name
Description
Position (detector pixels)
NIC1
Optimal center of Camera 1
162,100
NIC1-FIX
Geometric center of Camera 1
128,128
NIC2
Optimal center of Camera 2
149,160
NIC2-FIX
Geometric center of Camera 2
128,128
NIC2-CORON
Center of Coronographic Mask
NIC2-ACQ
Center of Mode 2 ACQ region
NIC3
Optimal center of Camera 3
140,135
NIC3-FIX
Geometric center of Camera 3
128,128
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Last updated: 07/24/97 15:31:29